Multi-axis interferometers generally divide a laser beam into several separate input beams, one for each axis of the interferometer. The separation between the input beams depends on the geometry of the interferometer and must provide measurement beams with sufficient separation for a determination of the pitch and yaw of the object or objects being measured. Generally, for accurate measurements of an object, the measurement beams must be parallel to each other to within a small angular tolerance that is often less than a few arcseconds.
Shearplate beam-splitters can generate separate input beams in interferometers. A shearplate beam-splitter is basically a plate of glass having two parallel surfaces. A light beam enters the shearplate beam-splitter at a first surface and is partially transmitted at a second surface. The transmitted portion of the beam forms a first input beam. The internally reflected portion subsequently reflects off the internal surfaces of the shearplate beam-splitter one or more times before exiting at the second face to form a second input beam. The two input beams remain parallel because the surfaces of the shearplate beam-splitter can be made parallel and flat to a high degree of accuracy.
A disadvantage of shearplate beam-splitters is that achieving larger separations between beams generally requires thicker and longer plates of glass. Accordingly, the shearplate beam-splitters can be too large for some applications.
A rhomboid/prism assembly is another optical system that can split a beam into two separated parallel beams. A rhomboid/prism assembly generally includes a rhomboid element and a prism attached to the rhomboid element. With one input scheme, a beam entering perpendicular to a first face of the prism is partially reflected at the interface between the prism and the rhomboid element. The reflected portion exits the prism and forms a first beam. The transmitted portion travels the length of the rhomboid element, reflects from a surface of the rhomboid element that is parallel to the interface with the prism, and exits the rhomboid element as a second beam, which is parallel to the first beam. The length, not the thickness, of a rhomboid element determines the separation between the beams. Accordingly, rhomboid/prism assemblies do not have to increase in thickness to increase beam separation and therefore do not have the size problem associated with shearplate beam-splitters.
A disadvantage of rhomboid/prism assemblies is that manufacture of these assemblies from individual rhomboid and prism components requires the individual components to be fabricated with a very high degree of accuracy, and the process that glues the prism to the rhomboid element must be similarly accurate. These difficulties, particularly the requirement of accurate angles between the faces of the rhomboid and prism elements, make manufacture of rhomboid/prism assemblies problematic when the exiting beams must be parallel to within a few arcseconds.
For an interferometer, a shearplate beam-splitter and/or a rhomboid/prism assembly can be mounted with a polarizing beam-splitter (PBS), a reference mirror or mirrors, and other optical elements on a rigid base. The base that holds the separate components such as the PBS and reference mirrors in alignment must be made of a physically and thermally stable material to ensure stable interferometer measurements.
A disadvantage of mounting the critical components separately in this fashion is the measurement instability that still arises from thermal expansion of the mounting structure even though the mounting structure is made of a stable material.
In accordance with an aspect of the invention, the manufacture of a rhomboid assembly starts with parallel plates of glass typically having optical coatings on one or more surface. The parallel plates are glued together before the cutting, grinding, and polishing that forms additional optical surfaces of the rhomboid assembly. The component elements of the rhomboid assembly are automatically matched to each other and not subject to the difficulties encountered when attaching separate preformed components because optical surfaces of the components are formed after the elements are rigidly attached. The rhomboid assembly thus manufactured solves the size problem associated with shearplate beam-splitters and can be extended to provide three or more separate beams. Additionally, the manufacturing method allows batch manufacture of multiple separate rhomboid assemblies from the same parallel plates.
One specific method in accordance with the invention manufactures a rhomboid assembly by forming a coating on a surface a first plate of glass, gluing the first plate to a second plate of glass with the coating between the first and second plate, making parallel cuts through the first and second plate of glass at an acute angle, and finishing some or all of the resulting parallel surfaces to optical tolerances. Before making the parallel cuts, one or more additional plate of glass can be glued with additional coatings between adjacent pair of plates. The parallel cuts cut through all the glued plates, and the resulting rhomboid assembly contains as many elements as there were plates glued together. Cutting and finishing an end of an assembly can convert one of the rhomboid elements into a prism.
In accordance with another aspect of the invention, an integrated beam handling optical structure can include one or more rhomboid assemblies and other optical elements that are optically attached into an integrated whole. Attaching the separate elements provides a compact and lightweight configuration with thermal stability that maintains the relative orientations of the surfaces of the optical elements. Accordingly, a compact interferometer with the integrated beam handling optics can provide measurement stability and a large number of measurement axes.
One specific embodiment increases measurement stability by placing a highly reflective (HR) coating directly on a quarter-wave plate or other element that is optically attached to a polarizing beam-splitter (PBS). The HR coating forms a reference mirror that has a stable position and orientation relative to the PBS. This eliminates possible relative motion between a PBS and reference mirrors and eliminates the need for a separate reference mirror.
A specific embodiment of the invention is an interferometer including: a PBS oriented to split an input beam into a reference beam and a measurement beam; a polarization-changing element such as a quarter-wave plate that is optically attached to a surface of the PBS and in a path of the reference beam; and a reflective coating on the polarization-changing element. The reflective coating reflects the reference beam back through the polarization-changing element and back into the PBS. Other elements such as a retroflector can be optically attached to the PBS so that the entire path of the reference beam up to output with the measurement beam is inside the integrated beam optics structure.
Another embodiment of the invention is an interferometer that includes a PBS and a first rhomboid assembly optically attached to the PBS. The first rhomboid assembly receives a first beam and splits the first beam into multiple input beams directed into the PBS. The interferometer can further include a second rhomboid assembly optically attached to the PBS. The second rhomboid assembly receives a second beam and splits the second beam into a multiple beams. One of the beams from the second rhomboid assembly is the first beam input to the first rhomboid assembly. Generally, the beams from the first rhomboid assembly are separated from each other along a first axis, and the beams from the second rhomboid assembly are separated from each other along a second axis that is perpendicular to the first axis.
The interferometer can still further include one or more optical elements optically attached to the PBS and the second rhomboid assembly. Each of these optical elements receives one of the other beams from the second rhomboid assembly and directs one or more beams toward the PBS. Each of these optical elements can be for example a rhomboid element or a further rhomboid assembly.
Still another embodiment of the invention is an interferometer including: a PBS oriented to split an input beam into a reference beam and a measurement beam; and a rhomboid element optically attached to the PBS and positioned to receive the measurement beam from the PBS. The rhomboid element shifts the position of the measurement beam to correspond to the position of a measurement reflector and can provide a separation between measurement beams that is greater than the PBS could otherwise accommodate. Generally, the interferometer also includes an extension to the PBS for the path of the reference beam, the extension having a length such that an optical path length of the reference beam through the extension matches an optical path length of the measurement beam. An extension can also or alternatively be provided when to match the optical path length of a reference beam to the optical path length of a measurement beam that traverses glass in the measurement reflector. These extensions can be part of the PBS or separate elements optically attached to the PBS.